The present invention relates to laser devices, and more particularly to a discharge excitation type short pulse laser device in which an electric discharge is caused in a variety of gases such as atomic gases, molecular gases, ionic gases, gaseous mixtures of them, metal vapors and vapors from volatile liquids for excitation to thereby produce a short pulse laser beam.
A conventional discharge excitation type short pulse laser device of this type is as shown in FIG. 1 In FIG. 1, reference numeral 1 designates a capacitor for storing energy for a main discharge; 2, a peaking capacitor; 3, a charging inductance; 4, a high voltage switch for starting an electric discharge comprising a thyratron; 5, a first main electrode arranged in a laser medium 14; 6, a second main electrode which is also arranged in the laser medium 14 so as to be spaced a predetermined distance from the first main electrode 5; 7, a main electric discharge which takes place between the two main electrodes 5 and 6; 8, a discharge gap for auxiliary ionization connected in series to the peaking capacitor 2; 9, ultraviolet rays formed in the discharge gap 8; and 10, a high voltage generating unit.
In the discharge excitation type short pulse laser device thus organized, after the capacitor 1 has been charged by the high voltage from the high voltage generating unit 10 through the charging inductance 3, the high voltage switch 4 is turned on to complete the loop of the capacitor 1, the peaking capacitor 2 and the high voltage switch 4, so that the peaking capacitor 2 is quickly charged by the pulse voltage. As shown in FIG. 1, the peaking capacitor 2 is connected in parallel to the first and second main electrodes 5 and 6. Therefore, as the charging of the peaking capacitor 2 progresses to increase the potential difference between the main electrodes 5 and 6, the dielectric breakdown of the laser medium 14 between the main electrodes 5 and 6 is caused, so that the main discharge 7 occurs therebetween. This circuit is so called "a capacitance shift type circuit", and it is extensively employed as a short pulse laser device as well as a conventional "LC inversion type circuit".
On the other hand, in a sort pulse laser device such as an ordinary "TEA CO.sub.2 laser" or "excimer laser", its operating pressure is high, several atmospheres for instance, and therefore the above-described electric discharge is liable to converge; that is, the laser output is liable to decrease. In order to overcome this difficulty by providing a spatially uniform main discharge, a method has been employed in which a preliminary electric discharge is carried out to uniformly scatter discharging electron seeds in advance in the area where the main discharge occurs. In the laser device as shown in FIG. 1, the preliminary ionization is carried out by the ultraviolet rays 9 which is produced in the discharging gap 8 connected in series to the peaking capacitor 2.
With the above described first example of the conventional discharge excitation type short pulse laser device shown in FIG. 1, the amount of preliminary ionization depends on the circuit for performing the main discharge, and therefore it is difficult to adjust the time when the amount of preliminary ionization becomes maximum. Furthermore, since a large quantity of electric charges passes through the discharging gap, an intense spark discharge occurs in the discharge gap. As a result, not only the energy but the electrodes are consumed uneconomically, and further impure gases are produced.
FIG. 2 shows a second example of the conventional discharge excitation type short pulse laser device in which similarly as in the laser device shown in FIG. 1, the preliminary ionization is performed to make the main discharge uniform. In FIG. 2, those components which have been previously described with reference to FIG. 1 have therefore been similarly numbered.
With such a laser device as shown in FIG. 2, a second main electrode 6 has a number of holes, and a dielectric material 15 is laid between the porous main electrode 6 and an auxiliary electrode 16 to form a capacitor which is connected in parallel to peaking capacitor 2.
When, in the laser device of FIG. 2, the high voltage switch 4 is turned on, the voltage across the peaking capacitor 2, i.e., between the second main electrode 6 and the auxiliary electrode 16, rises as shown in FIG. 7(A) similarly as in the case of the laser device of FIG. 1.
On the other hand, a current whose waveform is as shown in FIG. 7(B) flows from the capacitor 1 to the peaking capacitor 2 and the capacitor made up of the second electrode 6 and the auxiliary electrode 16, to charge these capacitors. The rate of flow of the current depends on the composite capacitance of the capacitor 1, the peaking capacitor 2 and the capacitor made up of the second main electrode 6 and the auxiliary electrode 16, and on the stray inductance of the circuit. And the current oscillates in each 50 to 100 ns periods as shown in FIG. 7(c). In the porous region of the second main electrode 6, a creeping discharge is caused by the above-described charging current resulting in the preliminary ionization.
In the conventional laser device shown in FIG. 2, similarly as in the case of the conventional laser device of FIG. 1, the preliminary ionization depends mainly on the circuit of the main discharge, and therefore it is difficult to adjust the time when the amount of preliminary ionization becomes maximum, and the electrons formed by the preliminary ionization may disappear. Therefore, a smaller part of the number of electrons formed by the preliminary ionization is used for the main discharge. In addition, the preliminary ionization is carried out only once. Accordingly, the electrons formed by the preliminary ionization is liable to become nonuniform in distribution. As a result, it becomes difficult to make the main discharge uniform, resulting in the decrease of the laser output.